\begin{tikzpicture}[>=Stealth]
    \draw (0, 0) -- (0, 3) -- (3, 3) -- (3,0) -- (0, 0);
    \draw (0, 1) -- (1, 3) -- (3, 2) -- (2, 0) -- (0, 1);
    \draw[pattern={Lines[angle=35]}] (0, 3) -- (0, 1.6) -- (0.8, 3) -- (0, 3);

    \node [above] at (1.5, 3) {$a$};
    \node [rotate=-30] at (2.3, 1) {$b$};
    \node [below] at (1, 0) {$n$};
    \node [right] at (3, 1) {$n$};
    \node [below] at (2.5, 0) {$m$};
    \draw (2.3, 0) arc (0:60:0.3) +(0.3, 0) node {$x$};
\end{tikzpicture}
